agentHost: render completed reasoning fallback

[Silence (silence48)]

Summary

• render completed Copilot CLI reasoning when no non-empty assistant.reasoning_delta created a live reasoning part
• use assistant.message.reasoningText as the final fallback, matching restored-session behavior
• preserve parent/subagent routing and existing tool-round response-part boundaries
• suppress completed-event duplication after a successful delta stream and ignore empty transport deltas

Fixes #332053

Why

The live Agent Host adapter already renders assistant.reasoning_delta, but its assistant.reasoning handler only logs the completed text. The assistant.message handler likewise treats reasoningText as history-only even though the restored-session mapper renders it. Providers or SDK paths that emit only the completed form therefore persist a readable summary that never appears live.

The existing reasoningPartIds map already tracks the current parent/subagent reasoning scope and is cleared at tool-call boundaries. This change uses that state as a fail-safe fallback gate instead of adding a parallel lifecycle tracker.

Tests

Added focused regression coverage for:

• completed reasoning after no non-empty delta
• no duplicate after streamed reasoning
• assistant.message.reasoningText fallback ordering before markdown
• completed subagent reasoning routing

git diff --check passes. The full repository test suite is left to CI because this branch was prepared from a dependency-free sparse checkout.

Related: #312559, #328146, #329873, #330279.

AI-assisted implementation and description.

[Copilot]

Pull request overview

Adds live fallback rendering for completed Copilot CLI reasoning when streaming deltas are unavailable.

Changes:

• Renders completed reasoning from reasoning events and message history fields.
• Ignores empty reasoning deltas and preserves subagent routing.
• Adds focused regression tests.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
copilotAgentSession.ts	Implements completed-reasoning fallback behavior.
copilotAgentSession.test.ts	Covers fallback, deduplication, ordering, and routing.

---

💡 Add a code-review agent skill or configure MCP servers for context-aware, tailored reviews. Learn more in the docs.

[Silence (silence48)]

@microsoft-github-policy-service agree I have sole ownership of intellectual property rights to my Submissions and I am not making Submissions in the course of work for my employer.

[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated no new comments.

Suppressed comments (1)

src/vs/platform/agentHost/node/copilot/copilotAgentSession.ts:1685

• The SDK's documented turn order is assistant.reasoning_delta → assistant.reasoning → assistant.message, but the completed handler removes the reasoning ID from this map before the message arrives. The subsequent message therefore sees no streamed state and emits reasoningText again, duplicating both streamed and completion-only reasoning in the normal event order. Keep prior rendered/completed reasoning state through the message boundary (and correlate it with the message fallback), then add a regression using the documented order.

		const streamedSinceLastMessage = turn.pendingReasoningDeltaIds.get(reasoningScope);
		turn.pendingReasoningDeltaIds.delete(reasoningScope);
		turn.pendingMessageReasoningFallbacks.delete(reasoningScope);
		if (!content || streamedSinceLastMessage?.size) {

[Mohammad javad Dianat (dianatofficial)]

Approved. The fix is minimal and correct.